In a glass or ceramic bowl, stir together the cornstarch, sugar and salt. Mix in the water. Heat in the microwave until the mixture is thick and clear, about 2 minutes at full power. Set aside to cool slightly.
Beat egg whites in a large glass or metal bowl until foamy. Gradually pour in the sugar mixture while continuing to whip the egg whites until thick and stiff enough to hold a peak. Cover pie filling completely with meringue, sealing to the edges. Bake as directed in the recipe, or in a preheated 450 degrees F (220 degrees C) oven until golden brown, 7 to 8 minutes.
